Aux Input Problem

I have a Pioneer deh-2100 head unit with an aux in for RCA cables. I've hooked up an MP3 player with an RCA to headphone jack cable, but the sound only comes out of the rear speakers. It plays over the radio and the CD player, but I can only adjust the sound with the MP3 player. Can someone help me out with boosting the audio and playing in all of the speakers?